1. Field of the Invention
The present invention relates to an electron gun in which distortion of an object point diameter etc. of an electron beam is reduced and a cathode ray tube provided with such an electron gun.
2. Description of the Related Art
In a color cathode ray tube (CRT), as shown in for example FIG. 1, a vacuum envelope is constituted by a panel 10, a funnel 11, and a neck 12. The panel 10 and the funnel 11 are bonded by frit glass 13. A phosphor surface coated with phosphors emitting lights of blue, green, and red is formed on an inner surface of the panel 10, and a color selection mask 21 is arranged close to this phosphor surface. An electron beam from an electron gun 30 accommodated in the neck 12 is deflected to a predetermined direction by a not illustrated deflection yokes. The beam passes through the color selection mask 21, reaches the phosphor surface formed on the inner surface of the panel 10, excites the predetermined phosphors, and makes them emit the light.
This electron gun 30 is a unipotential type and causes convergence of electrons emitted from red, green, and blue cathodes K1 to K3 onto the phosphor surface by the action of a prefocus lens constituted by a first grid G.sub.1, a second grid G.sub.2, and a third grid G.sub.3 and a main lens constituted by a third grid G.sub.3, a fourth grid G.sub.4, and a fifth grid G.sub.5. These grids are arranged so as to be coaxial.
An enlarged view of a conventional cathode K and prefocus lens system is given in FIG. 2. The first grid G.sub.1, the second grid G.sub.2, and the third grid G.sub.3 are assembled so as to be coaxial by fixing them to predetermined jigs and inserting their respectively provided connection pins CP.sub.1 to CP.sub.3 in melted bead glass BG. In the past, the precision of this assembly process has been a problem.
A model of the cathode lens system is shown in FIG. 3. The electron beam EB emitted from the cathode K is focused sharply to form a beam by the action of the first grid G.sub.1 and the second grid G.sub.2 and then expands. The smallest diameter of the beam will be referred to as an object point diameter .O slashed.c. This object point diameter .O slashed.c becomes an effective image of the convergence lens after this. Any axial deviation of an aperture diameter .O slashed..sub.1 of the first grid G.sub.1 and .O slashed..sub.2 of the second grid G.sub.2 and lack of parallelism of the distance d.sub.12 between the first grid G.sub.1 and the second grid G.sub.2 exert an adverse influence upon the shape and distortion of the object point diameter .O slashed.c formed by the cathode lens. In order to secure sufficient precision, steps have been taken with respect to the jigs and steps have been taken such as provision of guide holes in the first grid G.sub.1, second grid G.sub.2, etc., but they have not yet been sufficient.
An example of an electron gun designed to improve the precision of assembly is shown in FIGS. 4 and 5. This electron gun has a structure in which the first grid G.sub.1 and the second grid G.sub.2 are fixed in place via a ring-like spacer 33 made of an insulating material. In the assembly of this electron gun, first, as shown in FIG. 5, which is an enlarged view of the part surrounded by a circle in FIG. 4, metallized layers 33a composed of Mo--Mn are formed by sintering on the two end surfaces of a ring-like spacer 33 constituted by an insulator mainly composed of Al.sub.2 O.sub.3, then a Ni plating 33b is applied. Silver solder 33c is interposed between this spacer 33 and the first grid G.sub.1 and the second grid G.sub.2, then the assembly is mounted on a jig for positioning the spacer 33, the first grid G.sub.1, and the second grid G.sub.2 is subject to heat treatment in a hydrogen furnace so as to bond the parts together.
In the production of this electron gun, however, nickel plating and brazing are carried out after metallizing the ceramic and other numerous steps are performed, so the process becomes expensive. In effect as well, only improvement of the beam spot diameter due to an improvement of the dimensional precision can be expected.
On another matter, as a cut-off voltage E.sub.kco of the electron gun, the following Equation (1) has been known. EQU E.sub.kco .alpha.(.O slashed..sub.1.sup.3 /d.sub.10 .multidot.d.sub.12t .multidot.t.sub.1).multidot.t (1)
The symbols in the equation are the same as the symbols shown in FIG. 3, that is, .O slashed..sub.1 is the aperture diameter of the first grid G.sub.1, d.sub.10 is a distance between the cathode K and the first grid G.sub.1, d.sub.12 is a distance between the first grid G.sub.1 and the second grid G.sub.2, and t.sub.1 is a plate thickness of the first grid G.sub.1. The higher the cut-off voltage, the larger the number of white and black gradations and thus the better the quality of the image. It is seen from this equation that the smaller the thickness t.sub.1 of the first grid G.sub.1, the higher the cut-off voltage E.sub.kco.
However, the grid is formed by a metal plate such as a stainless steel plate, therefore the thickness thereof is about 50 .mu.m. It is difficult to reduce the thickness more than this so long as the grid is formed by a metal plate.
On the other hand, the bead glass BG shown in FIG. 2 is assembled integrally also with the fourth grid G.sub.4 and the fifth grid G.sub.5. When the electron gun is used, for example, an anode voltage (20 to 30 kV) is applied to the third grid G.sub.3 and the fifth grid G.sub.5, and a medium voltage of 10 to 5 kV is applied to the fourth grid G.sub.4. These are higher in comparison with the about 700 V of the second grid G.sub.2, therefore the bead glass BG is charged and also the first grid G.sub.1 connection pin portion CP.sub.1 is given a high voltage. In order to eliminate the effect of charging this bead glass BG with a high voltage, it is necessary to make the distance d.sub.12 between the first grid G.sub.1 and the second grid G.sub.2 sufficiently smaller with respect to the outer diameter of the first grid G.sub.1 and the second grid G.sub.2. By rule of thumb, it is necessary to make the same about 1/10.
The same is true also for the electron gun shown in FIG. 4. Even when the voltage applied to the second grid G.sub.2 is set to 200 V, again, to eliminate the effect of charging the spacer (insulator) 33, as the shape of the second grid G.sub.2 shown in FIG. 2, it is necessary to adopt a configuration in which d.sub.12 is made sufficiently narrow by forming a convex portion on the first grid G.sub.1 side. There are therefore restrictions on the structure.